Good or even great food for two days should be easy to find in both Oslo and Copenhagen.

While not very familiar with Copenhagen I would think Oslo would win easily on nature, but I guess it depends on the nature of your interest.

With regards to architecture Copenhagen probarly in diversity, but in Oslo you can experience an inner city in process of being completely changed. Barcode, Opera house and Tjuvholmen are already finished but there's also a lot building going on.

(As others have said, you should probarly have a few more days for Bergen as it's a great base for fjords and such - however one option is to fly into Bergen and the take the train trip "Norway in a nutshell" to Oslo or vice versa.)